LIGHT A VITAL AGENCY. 231 



This has a paradoxical air, when we reflect on the para- 

 mount importance of light among vital agencies. In dark- 

 ness the infusoria, it is said, will not develop. In darkness 

 the plant withers. Tiy to rear a plant in darkness, and no 

 amount of heat, air, or moisture (the other vital agencies) 

 will stimulate it to the processes of real growth. Deprived 

 of light, it is deprived of food, and the possibility of food. 

 It then lives entirely on its own substance, like a starving 

 animal ; the store of food which was in the seed is used up, 

 but no new food can be assimilated from without. Nay, if 

 the experiment be carefully conducted, you will find that 

 your plant in darkness, in spite of apparent growth, has 

 really lost some of its substance, instead of increasing it ; 

 weighing less, when dried, than the dry seed from which it 

 issued. Science has proved that it is in light, and in light 

 alone, that plants deoxidise carbonic acid — setting free the 

 oxygen, which can then be breathed by animals, and in thus 

 setting free the oxygen, releasing the carbon, which nourishes 

 the tissues of the plant. It was thought (and is still printed 

 in many text-books) that the gi^een parts caused the liberation 

 of oxygen in light ; but Mulder corrects this, saying that the 

 parts do not liberate the oxygen because they are green, 

 but become green in the process.* Rear the plant in dark- 

 ness, and its leaves will be pale ; bring it into sunlight, and 

 these pale leaves instantly decompose carbonic acid, and 

 assume a green tint. 



The history of our knowledge of the relation between 



* Mulder : Versuch einer Pki/siol. Chemie, 185L A translation of this valu- 

 able work was published under the auspices of the late Professor Johnston. 



